This invention relates to a fixture for measuring the size and shape of an article. More particularly, the invention concerns a fixture having a solid core and an outer skin that is formed over the solid core, the outer skin being machined for measuring the size and shape of the article.
Fixtures on which a workpiece, such as a sheet of automobile windshield glass, can be placed for the measurement of the size and shape of the workpiece are well known. A portion of the fixture is typically machined to have the desired shape of the workpiece to be measured, and to have the desired size and/or markings which indicate the desired size of the workpiece to be measured. When the workpiece is placed on the fixture, the deviations between the desired size and shape of the workpiece and the actual size and shape of the workpiece are quantified.
The material and manufacturing costs of high precision fixtures which function well over an extended time and over varying environmental conditions can be extremely expensive. Homogeneous solid core fixtures, such as mahogany, can expand and contract due to environmental conditions, such as humidity and temperature, and also change with age as the wood dries out. This affects the measurements obtained using the fixture and makes it unsuitable for high-precision applications. Solid fixtures are also heavy and may require forklifts or other special modes of transportation to move them, as occurs quite often in plants that manufacture a plurality of different workpieces, and thereby have a plurality of different fixtures.
One solution to this problem is to use the solid core as a general shape during the fabrication process and apply a skin over the solid core. Optimally, the skin material is chosen to exhibit very little change due to environmental conditions or aging. The skin can then be machined to the desired size and shape and its core removed. The fixture is then the skin which exhibits very little change in size and shape due to environmental conditions or aging. A disadvantage of this type of fixture is that the skin, without the core, lacks strength and structural rigidity. The skin fixture is susceptible to changes in shape or breakage due to the weight of the articles being measured, transport of the fixture and other factors indigenous to a manufacturing facility.
One remedy to help compensate for the fragile nature of the hollow core fixture is to add a support structure under the skin such as beams or a honeycomb structure. Such a support structure should be made of materials that will not change the shape of the skin due to environmental changes or aging. The support structure provides greater rigidity to the skin. However, additional steps and greater complexity of the fabrication process to add the support structure greatly increases the cost of this type of fixture.
Accordingly, a need exists for a fixture that maintains it shape with changes in environmental conditions and aging, yet is lightweight and is quick and inexpensive to fabricate. Therefore, one object of the present invention is to provide a fixture for measuring the size and shape of a workpiece, that is negligibly effected by changing environmental conditions and aging, is lightweight, and is quick and inexpensive to fabricate.
In accordance with the present invention a fixture is provided for measuring the size and shape of a workpiece. The fixture comprises a solid plug core that provides the general shape of the fixture during the fabrication process and is maintained in the fixture to provide additional strength and rigidity during usage of the fixture. The fixture includes a skin, comprising one or more layers for covering the plug core. The fixture also includes a surface material that is applied to a portion of the skin. The surface material is machined to meet the requirements for measurement of the particular workpiece(s) for which it is designed.
A feature of the present invention is that a fixture can be fabricated quickly and inexpensively. The plug core is used in the fabrication process and is not removed. The skin can be made in a one step process where the multiple skin layers are applied and a single curing period is used during which the skin layers are bonded. The plug core and skin only (without the surface material) provide the general shape of the fixture, thus they have very loose tolerances. Only the surface material needs to be machined to meet the required tolerances, thus permitting the fixture to be fabricated quickly and inexpensively.
Another feature is that the fixture can be made of relatively lightweight materials and yet have good strength and rigidity. The plug core provides strength and rigidity to the fixture. The skin can include a skin core which provides additional strength and rigidity in the skin layer, independent of the plug core. This enables the fixture to maintain good stability even when the plug core is made of an inexpensive, light weight material which is unstable or less stable than is required of the surface material.